Sterilizable packages and desiccant and fumigant packages of open cell microporous film

ABSTRACT

DESICCANT OR FUMIGANT PACKAGING PROVIDES A HIGHLY DESIRABLE END USE FOR CERTAIN OPEN-CELLED MICRROPOROUS FILMS BECAUSE OF THE PORE SIZE AND DISTRIBUTION WHICH IN THE DESICCANT PACKAGING APPLICATION ALLOWS RAPID TRANSPORT OF MOISTURE VAPOR INTO THE PACKAGE AND IN FUMIGANT PACKAGING SITUATIONS, PERMITS A SLOW RELEASE (EVAPORATION) OF A SOLID OR LIQUID FUMIGANT THESE FLEXIBLE HIGH STRENGTH FILMS NOT ONLY PREVENT GROSS CONTAMINATION OF SURROUNDINGS BUT ALSO POSSESS GOOD RESISTANCE TO HEAT, WATER AND MANY SOLVENTS STERILE PACKAGING IS A HIGHLY DESIRABLE END USE FOR CERTAIN OPEN-CELL MICROPOROUS FILMS BECAUSE OF THEIR HIGH LEVEL OF GASEOUS STERILANT TRANSMISSION; MOISTURE INSENSITIVITY AND BACTERIA IMPREMEABILITY.

United States Patent 6 U5. Cl. 161l59 19 Claims ABSTRACT F THEDHSCLGSURE Desiccant or fumigant packaging provides a highly desirableend use for certain open-celled microporous films because of the poresize and distribution which, in the desiccant packaging application,allows rapid transport of moisture vapor into the package and infumigant packaging situations, permits a slow release (evaporation) of asolid or liquid fumigant.

These fiexible, high strength films not only prevent gross contaminationof surroundings but also possess good resistance to heat, water, andmany solvents.

Sterile packaging is a highly desirable end use for certain open-cellmicroporous films because of their high level of gaseous sterilanttransmission; moisture insensitivity and bacteria impermeability.

BACKGROUND OF THE INVENTION Field of the invention The present inventionrelates to improved desiccant or fumigant packages by the use of certainopen-celled microporous films.

The present invention relates to improved sterile packaging productsthrough the use of certain micro-porous films.

Description of the prior art It is often advantageous to package andstore certain materials with a dehydrating and/or fumigating agent toprevent deleterious changes which could occur as a result of moistureand/or insect contamination. Often it is merely highly desirable to beable to obtain a moisture free product be it liquid, gas, or solid.

In dehydration operations, desiccating agents are rarely added directlyto the material or atmosphere from which water is to be extracted forthe agent is usually in the form or" very small granules or a finepowder to increase the surface area and thereby the elfectiveness of theagent. Direct addition of the agent would, of course, result in grosscontamination of the moisture containing region; usually a highlyundesirable situation. Moreover, most drying agents act vigorously whenfirst ex posed to the atmosphere but soon become spent or saturated sothat their effectiveness is short-lived and varies greatly in intensity.

In order to obviate this problem, it has been quite common in the art toenclose the granulated or powdered dehydrating agent Within a container,capsule, or coating of material which is preferably inert chemically butwhich is capable of absorbing and transmitting moisture. These devicesare often constructed to be hollow, elongated members and can be formedof some perforated non-corrosive metal but are more often non-perforatedcapsules composed of pliant, gas-permeable sheet material such as paper,casein, cellophane, cellulose acetate, or laminates of paper and plasticfilm. The dehydrating action of the paper and/or plastic film. capsulesis of course extremely slow for although the desiccating material issuccessfully shielded from direct contact with the material oratmosphere to be dried; the moisture from the material must be absorbedby the capsule film; diffuse through the capsule Wall; and throughdesorption, be absorbed internally by the desiccant, i.e. the dryingagent serves to extract the moisture from the capsule covering ratherthan from the atmosphere or material being treated.

A disadvantage to the use of perforated containers to enclosedesiccating agents lies in the inevitable pulverizing of even aninitially granulated agent due to movement within the container. This inturn leads to a sitting of the agent out of the container through theperforations resulting in, if not an undesirable chemical reaction, acontaminated albeit moisture-free product.

Paper desiccant containers not only possess the sifting disadvantageassociated with the perforated containers as elucidated above, but alsolose their integrity quite quickly when exposed to liquids or evenexcessively moist, solid substances.

Soild, volatile repellants and insecticides are usually stored orshipped Within metal containers or films of regenerated cellulose;materials the organic vapors are incapable of permeating. When thefumigating effect is desired, the articles are either brought intodirect physical contact with the solid fumigant, usually a mostundesirable situation which can result in particulate fumigantcontamination, or contained within a package having on at least one sidean organic thermoplastic film through which the fumigant vapors candiffuse. Examples of such films are polyethylene, polyvinyl chloride,vinyl chloride-vinyl acetate copolymers, vinylidene chloride copolymerswith vinyl chloride or acrylonitrile or ethyl acrylate, ethyl cellulose,cellulose acetate, and the like. Inherent in the use of these films isthe extremely slow vapor release realized as a result of the multiphasedvapor release process, i.e., vapor sublimation; film absorption; filmdiffusion; and finally vapor desorption.

Sterilization of medical, surgical and dental instruments, dressings,precision tools and the like is usually accomplished by one or threebasic methods.

The first involves placing all of the items in a sterilization chamber;introducing the sterilant; and thence, individually removing andwrapping the items for storage as illustrated in US. Pat. No. 3,163,494.This process necessitates handling the items after they have beensterilized, thereby assuming the undesirable risk of introducingcontaminates.

The second sterilization technique utilizes an instrument and/orappliance container such as described in US. Pat. No. 3,437,423 in whichthe items are placed during the sterilization process and in which theyare later stored. These containers are usually constructed from a metalor a dimensionally stable, heat resistant, thermoplastic material andare open-top, tray-like structures almost always provided With drainageholes to eliminate condensate. As a result of this open construction,the sterilized items maintain this aseptic condition for only a shortperiod of time.

The third method, a gas sterilization process, involves sealing eachindividual item in a plastic film or paper package; evacuating the airfrom this sealed package; and introducing an ethylene oxide mixture;e.g., a 12% ethylene oxide-88% trichlorofiuouromethane mixture, and asmall amount of water vapor. A relative humidity of 3540% should existin the sterilizing chamber at 25 C. for effective kill. A relativehumidity of 35-40% should exist in the sterilizer charge (as measured atroom temperature) for effective :kill.

The sterilization is accomplished at approximately to F. Subsequentethylene oxide removal (evacuation) and air purge is required. Theseevacuations require considerable time for gas transport and must be slowfor minimum package ruptures when used with films with only diffusioncapabilities. With films, original equipment packagers will often use an8, 12, or more hour sterilization cycle followed by an aeration periodthat could last from 48 hours to many days depending upon the type ofproduct sterilized.

Ethylene oxide is a very toxic, flammable gas which forms an explosivemixture with air and has become a standard sterilant in hospitals onlybecause of the impracticability of steam sterilizing soft goods such asgauze, bandages, etc. A problem inherent in ethylene oxide use is thefact that certain materials such as rubber and plastics, upon longexposure to the gas such as is necessitated when diffusion controlledfilms are utilized in the sterilizing packages, will absorb the gas,thereby requiring extremely long degassing periods to rid the item ofany residual ethylene oxide before use.

Also to be noted is the fact that though the diffusion rate of ethyleneoxide gas is slow through the plastic packaging films, the necessity ofintroducing a small amount of water vapor for effective kill, which hasan even slower diffusion rate than the gas, seriously hinders productionin commercial sterilization processes.

Film packages, as opposed to paper or coated paper packages, may also besteam sterilized in autoclaves at temperatures of from 230 F. to 270 F.,usually from 230 to about 240 F., but still retain the inherentdisadvantage of being vapor-diffusion controlled.

Commercial sterile film packages present storage problems for thegas/vapor pressure trapped within the package can only be relieved bygas-vapor difiusion through the plastic film, a slow process at best.Attempts to compress the packages for more efficient storage, onlyresult in film or seal ruptures.

Highly porous papers are undesirable in many sterile packagingsituations for, although they do permit rapid gas sterilization, theyare neither completely moisture insensitive nor bacteria impermeable.These factors, of course, severely limit post sterilization shelf lifefor aseptic condition of packaged contents.

In an attempt to improve the barrier characteristics of paper, apolymeric coating is often applied. These coatings, while successfullydecreasing the papers sensitivity to moisture, are, at best, animperfect solution for although they can prevent bacteria frompenetrating directly, they do not deter the bacteria from growingthrough the coating, thereby destroying the aseptic condition within thepackage. Extremely thick coatings will only destroy the advantage ofrapid gas transport sought for in the use of the paper initially.

Another disadvantage to the use of paper as a sterile packaging materiallies in the tearing of fibers and paperdust which accompanies theopening of such a package. The cellulose fibers may contaminate thesterilized item thereby defeating the purpose of using aseptic techniqueis dispensing sterile products.

Heretofore, porous films have been produced which possess a microporous,open-celled structure, and which are also characterized by a reducedbulk density. Films possessing this microporous structure are described,for example, in US. Pat. No. 3,426,754, which patent is assigned to theassignee of the present invention. The preferred method of preparationdescribed therein involves drawing or stretching at ambienttemperatures, i.e., cold drawing, a crystalline, elastic starting filmin an amount of about to 300 percent of its original length, withsubsequent stabilization by heat setting of the drawn film under atension such that the film is not free to shrink or can shrink only to alimited extent.

While the above-described microporous or void-containing film of theprior art is useful in this invention the search has continued for newprocesses able to produce open-celled microporous films having a greaternumber of pores, a more uniform pore concentration or distribution, alarger total pore area, and better thermal stability of the porous orvoidy film, e.g., for use in applications that call for repeated hightemperature sterilizations.

4 SUMMARY OF THE INVENTION Accordingly, an object of the presentinvention is to provide an improved desiccant and/or fumigant packagingsystem by utilizing on at least one portion of the surface of thedesiccant/fumigant enclosures an opencelled microporous plastic filmcontaining interconnected pores of from ten to three thousand angstromswhich will permit rapid gas/vapor transport while maintaining animpermeable solids/water barrier.

Another object is to provide for desiccant packaging an open-cellmicroporous film which will maintain its integrity in non-solvent liquidmedia.

An additional object of the present invention is to provide a desiccantcontainer which, with a film either by itself or in combination withother materials, constituting a completely sealed, desiccant-containingenvelope, will realize a controlled drying action.

An object of the present invention is to provide for improved sterilepackaging systems, a breathable waterproof film containing theinterconnected pores of from 10 to 3,000 A. which, by exceedingindustrial standards for sterile packaging films, will yieldsignificantly shortened gas sterilization cycles; i.e., permit rapidgas/vapor passage while excluding bacteria and other organisms when usedby itself or in combination with other materials will form a sterilized,shrink-fit package when used in a steam autoclave.

Other and further objects of the present invention will be apparent tothose skilled in the art from the following:

DETAILED DESCRIPTION OF THE INVENTION Porous or cellular films can beclassified into two general types: one type in which the pores are notinterconnected, i.e., a closed-cell film, and the other type in whichthe pores are essentially interconnected through tortuous paths whichmay extend from one exterior surface or surface region to another, i.e.,an open-cell film. The porous films of the present invention are of thelatter type.

The microporous films useful in the present invention are alsocharacterized by a reduced bulk density, sometimes hereinafter referredto simply as a low density. That is, these microporous films have a bulkor overall density lower than the bulk density of corresponding filmscomposed of identical polymeric material but having no open-celled orother voidy structure. The term bulk density as used herein means theweight per unit of gross or geometric volume of the film, where grossvolume is determined by immersing a known weight of the film in a vesselpartly filled with mercury at 25 C. and atmospheric pressure. Thevolumetric rise in the level of mercury is a direct measure of the grossvolume. This method is known as the mercury volumenometer method, and isdescribed in the Encyclopedia of Chemical Technology, vol. 4, page 892(Interscience 1949).

Porous films have been produced which possess a microporous, open-celledstructure, and which are also characterized by a reduced bulk density.Films possessing this microporous structure are described, for example,in US. Pat. 3,426,754 which patent is assigned to the assignee of thepresent invention. The preferred method of preparation described thereininvolves drawing or stretching at ambient temperatures, i.e., colddrawing, a crystalline, elastic starting film in an amount of about 10to 300 percent of its original length, with subsequent stabilization byheat setting of the drawn film under a tension such that the film is notfree to shrink or can shrink only to a limited extent.

While the above described microporous or void-containing film of theprior art is useful in this invention the search has continued for newprocesses able to produce open-celled microporous films having a greaternumber of pores, a more uniform pore concentration or distribution, alarger total pore area, and better thermal stability of the porous orvoidy film. These properties are significant in applications such asfilter media where a large number of uniformly distributed pores arenecessary or highly desirable; and in applications such as breathablemedical dressings subject to high temperatures, e.g. sterilizationtemperatures, where thermal stability is necessary or highly desirable.

An improved process for preparing open-celled microporous polymer filmsfrom non-porous, crystalline, elastic polymer starting films,includes 1) cold stretching, i.e., cold drawing the elastic film untilporous surface regions or areas which are elongated normal orperpendicular to the stretch direction are formed, (2) hot stretching,i.e., hot drawing, the cold stretched film until fibrils and pores oropen cells which are elongated parallel to the stretch direction areformed, and thereafter (3) heating or heat setting the resulting porousfilm under tension, i.e., at substantially constant length, to impartstability to the film. Yet another process is similar to this processbut consolidates steps (2) and (3) into a continuous simultaneous, hotstretching-heat setting step, said step being carried out for a timesufficient to render the resulting microporous film substantially (lessthan about percent) shrink resistant.

The elastic starting film or precursor film is preferably prepared fromcrystalline polymers such as polypropylene by melt extruding the polymerinto a film, taking up the eXtrudate at a drawdown ratio giving anoriented film, and thereafter heating or annealing the oriented film ifnecessary to improve or enhance the initial crystallinity.

The essence of the improved processes is the discovery that thesequential cold stretching and hot stretching steps impart to theelastic film a unique open-celled structure which results inadvantageous properties, including improved porosity, improved thermalstability and a gain or enhancement of porosity when treated withcertain organic liquids such as perchloroethylene.

As determined by various morphological techniques or tests such aselectron microscopy, the microporous films of the improved process arecharacterized by a plurality of elongated, non-porous, interconnectingsurface regions or areas which have their axes of elongationsubstantially parallel. Substantially alternating with and defined bythese non-porous surface regions are a plurality of elongated, poroussurface regions which contain a plurality of parallel fibrils or fibrousthreads. These fibrils are connected at each of their ends to thenon-porous regions, and are substantially perpendicular to them. Betweenthe fibrils are the pores or open cells of the films utilized by thepresent invention. These surface pores or open cells are substantiallyinterconnected through tortuous paths or passageways which extend fromone surface region to another surface area or region.

With such a defined or organized morphological structure, the filmswhich are treated according to the instant process may have a greaterproportion of surface area that the pores cover, a greater number ofpores, and a more uniform distribution of pores, than previousmicroporous films. Further, the fibrils present in the films of thepresent invention are more drawn or oriented with respect to the rest ofthe polymer material in the film, and thus contribute to the higherthermal stability of the film.

In fact the total surface area per cubic centimeter of material of thefilms of this invention have a range of from 2 to about 200 squaremeters per cc. Preferably the range is from about 5 to about 100 squaremeters per cc. and most preferably from about 10 to about 80 squaremeters per cc. These values can be compared with normal pin-holed filmwhich has a total surface area per gram of about 0.1 square meter; paperand fabric which have values per gram of about 1.0 square meter andleather which has a value of about 1.6 square meters per cc.Additionally, the volume of space per volume of material ranges fromabout 0.05 to about 1.5 cubic centimeters per gram, preferably fromabout 0.1 to about 1.0 cubic centimeters per gram and most preferablyfrom 0.2 to about 0.85 cubic centimeters per gram. Ad-

ditional data to define the films of this invention relates to nitrogenflux measurements, whereby the microporous films have Q (or nitrogen)flux values in the range of from about 5 to 400 preferably about 50 to300. These values give an indication of porosity, with higher nitrogenflux values indicating higher levels of porosity.

Nitrogen flux may be calculated by mounting a film having a standardsurface area of 6.5 square centimeters in a standard membrane cellhaving a standard volume of 63 cubic centimeters. The cell ispressurized to a standard differential pressure (the pressure dropacross the film) of 200 pounds per square inch with nitrogen. The supplyof nitrogen is then closed off and the time required for the pressure todrop to a final differential pressure of 150 pounds per square inch asthe nitrogen permeates through the film is measured with a stop watch.The nitrogen flux, Q, in gram moles per square centimeter minute, isthen determined from the equation:

where At is the change in time measured in second and T is thetemperature of nitrogen in degrees Kelvin. The above equation is derivedfrom the gas law, PV=Z RT.

The microporous films used in the present invention are formed from astarting elastic film of crystalline, film-forming, polymers. Theseelastic films have an elastic recovery at zero recovery time(hereinafter defined) when subjected to a standard strain (extension) of50 percent at 25 C. and 65 percent relative humidity of at least about40 percent, preferably at least about 50 percent, and most preferably atleast about percent.

Elastic recovery as used herein is a measure of the ability of astructure or shaped article such as a film to return to its originalsize after being stretched, and may be calculated as follows:

Although a standard strain of 50 percent is used to identify the elasticproperties of the starting films, such strain is merely exemplary. Ingeneral, such starting films will have elastic recoveries higher atstrains less than 50 percent, and somewhat lower at strainssubstantially higher than 50 percent, as compared to their elasticrecovery at a 50 percent strain.

The starting elastic films will also have a percent crystallinity of atleast 20 percent, preferably at least 30 percent and most preferably atleast 50 percent, e.g., about 50 to percent, or more. Percentcrystallinity is determined by the X-ray method described by R. G. Quynnet al. in the Journal of Applied Polymer Science, vol. 2 No. 5 pp.166473 (1959). For a detailed discussion of crystallinity and itssignificance in polymers, see Polymers and Resins, Golding (D. VanNostrand, 1959).

Preferred suitable starting elastic films, as well as the preparationthereof, are further defined in British Pat. No. 1,198,695, publishedJuly 15, 1970. Other elastic films which may be suitable for thepractice of the present invention are described in British Pat. No.1,052,550, published Dec. 21, 1966 and are well known in the art.

The starting elastic films utilized in the preparation of themicroporous films of the present invention should be differentiated fromfilms formed from classical elastomers such as the natural and syntheticrubbers. With such classical elastomers the stress-stain behavior, andparticularly the stress-temperature relationship, is governed byentropy-mechanism of deformation (rubber elasticity). The positivetemperature coefiicient of the retractive force, i.e., decreasing stresswith decreasing temperature and complete loss of elastic properties atthe glass transition temperatures, are particularly consequences ofentropy-elasticity. The elasticity of the starting elastic filmsutilized herein, on the other hand, is of a different nature. Inqualitative thermodynamic experiments with these elastic starting films,increasing stress with decreasing temperature (negative temperaturecoefiicient) may be interpreted to mean that the elasticity of thesematerials is not governed by entropy effects but dependent upon anenergy term. More significantly, the starting elastic films have beenfound to retain their stretch properties at temperatures where normalentropyelasticity could no longer be operative. Thus, the stretchmechanism of the starting elastic films is though to be based onenergy-elasticity relationships, and these elastic films may then bereferred to as non-classical elastomers.

As stated, the starting elastic films employed in this invention aremade from a polymer of a type capable of developing a significant degreeof crystallinity, as contrasted with more conventional or classicaelastic materials such as the natural and synthetic rubbers which aresubstantially amorphous in their unstretched or tensionless state.

A significant group of polymers, i.e., synthetic resinous materials, towhich this invention may be applied are the olefin polymers, e. g.,polyethylene, polypropylene, poly-3-methyl butene-l, poly-4-methylpentene-l, as well as copolymers of propylene, 3-methyl butene-l,4-methyl pentene-l, or ethylene with each other or with minor amounts ofother olefins, e.g., copolymers of propylene and ethylene, copolymers ofa major amount of B-methyl butene-l and a minor amount of a straightchain n-alkene such as n-octene-l, n-hexadecene-l, n-octadecene-l orother relatively long chain alkenes, as well as copolymers of 3-methylpentene1 and any of the same n-alkenes mentioned previously inconnection with 3-methyl butene-l. These polymers in the form of filmshould generally have a percent crystallinity of at least 20 percent,preferably at least 30 percent, and most preferably about 50 percent to90 percent or higher.

For example, a film-forming homopolymer of polypropylene may beemployed. When propylene homopolymers are contemplated, it is preferredto employ an isotactic polypropylene having a percent crystallinity asindicated above, a weight average molecular Weight ranging from about100,000 to 750,000 preferably about 200,000 to 500,000 and a melt index(ASTM-1958D- 1238-57T, Part 9, page 38) from about 0.1 to about 75,preferably about 0.5 to 30, so as to give a final film product havingthe requisite physical properties.

While the present disclosure and examples are directed primarily to theaforesaid olefin polymers, the invention also contemplates the highmolecular weight acetal, e.g., oxymethylene, polymers. While both acetalhomopolymers and copolymers are contemplated, the preferred acetalpolymer is a random oxymethylene copolymer, one which contains recurringoxymethylene, i.e.,

units interspersed with OR-- groups in the main polymer chain where R isa divalent radical containing at least two carbon atoms directly linkedto each other and positioned in the chain between the two valences, withany substituents on said R radical being inert, that is, those which donot include interfering functional groups and which will not induceundesirable reactions, and wherein a major amount of the OR units existas single units attached to oxymethylene groups on each side. Examplesof preferred polymers include copolymers of trioxane and cyclic etherscontaining at least two adjacent carbon atoms such as the copolymersdisclosed in US. Pat. 3,027,352 of Walling et al. These polymers in filmform may also have a crystallinity of at least 20 percent, preferably atleast 30 percent, and most preferably at least 50 percent, e.g. 50 to 60percent or higher. Further, these polymers have a melting point of atleast C., and a number average molecular weight of at least 10,000. Fora more detailed discussion of acetal and oxymethylene polymers, seeFormaldehyde, Walker, pp. -191, (Reinhold 1964).

Other relatively crystalline polymers to which the invention may beapplied are the polyalkylene sulfides such as polymethylene sulfide andpolyethylene sulfide, the polylarylene oxides such as polyphenyleneoxide, the polyamides such as polyhexamethylene adipamide (nylon 66) andpolycaprolactam (nylon 6), and polyesters such as polyethyleneterephthalate, all of which are well known in the art and need not bedescribed further herein for sake of brevity.

The types of apparatus suitable for forming the starting elastic filmsof this invention are well known in the art.

For example, a conventional film extruder equipped with a shallowchannel metering screw and coat hanger die, is satisfactory. Generally,the resin is introduced into a hopper of the extruder which contains ascrew and a jacket fitted with heating elements. The resin is melted andtransferred by the screw to the die from which it is extruded through aslot in the form of a film from which it is drawn by a take-up orcasting roll. More than one take-up roll in various combinations orstages may be used. The die opening or slot width may be in the range,for example, of about 10 to 200 mils.

Using this type of apparatus, film may be extruded at a drawdown ratioof about 20:1 to 200:1, preferably 50:1 to 150:1.

The terms drawdown ratio or, more simply, draw ratio, as used herein isthe ratio of the film wind-up or take-up speed to the speed of the filmissuing at the extrusion die.

The melt temperature for film extrusion, is in general, no higher thanabout 100 C. above the melting point of the polymer and no lower thanabout 10 C. above the melting point of the polymer.

For example, polypropylene may be extruded at a melt temperature ofabout C. to 270 C., preferably 200 C. to 240 C. Polyethylene may beextruded at a melt temperature of about 175 C. to 225 C., while acetalpolymers, e.g., those of the type disclosed in US. Pat. 3,027,352 may beextruded at a melt temperature of about C. to 235 0., preferably C. to215 C.

The extrusion operation is preferably carried out with rapid cooling andrapid drawdown in order to obtain maximum elasticity. This may beaccomplished by having the take-up roll relatively close to theextrusion slot, e.g., within two inches and, preferably, within oneinch. An air knife operating at temperatures between, for example 0 C.and 40 C., may be employed within one inch of the slot to quench, i.e.,quickly cool and solidify the film. The take-up roll may be rotated, forexample, at a speed of 10 to 100 ft./min. preferably to 500 ft./min.

While the above description has been directed to slit die extrusionmethods, an alternative method of forming the starting elastic filmscontemplated by this invention is the blown film extrusion methodwherein a hopper and an extruder are employed which are substantiallythe same as in the slot extruder described above. From the extruder, themelt enters a die from which it is extruded through a circular slot toform a tubular film having an initial diameter D Air enters the systemthrough an inlet into the interior of said tubular film and has theeffect of blowing up the diameter of the tubular film to a diameter DMeans such as air rings may also be provided for directing the air aboutthe exterior of extruded tubular film so as to provide quick andeffective cooling. Means such as cooling mandrel may be used to cool theinterior of the tubular film. After a short distance during which thefilm is allowed to completely cool and harden, it is wound up on atake-up roll.

Using the blown film method, the drawdown ratio is preferably 20:1 to200:1, the slot opening 10 to 200 mils,

the D /D ratio, for example, 0.5 to 6.0 and preferably about 1.0 toabout 2.5, and the take-up speed, for example, 30 to 700 ft./ min. Themelt temperature may be within the ranges given previously for straightslot extrusion.

The extruded fiim may then be initially heat treated or annealed inorder to improve crystal structure, e.g., by increasing the size of thecrystallites and removing imperfections therein.

In order to render the precursor or starting film microporous, it issubject to a process generally comprising the steps of stretching andheat setting the starting films. Preferably the process comprises eitherthe consecutive steps of cold stretching, hot stretching and heatsetting or the steps of cold stretching and simultaneously hotstretching and heat setting the precursor film. Other variations on thisprocess (such as elimination of the hot stretching step) can be carriedout resulting in microporous films which, although slightly inferior tothose films made by the cold stretch-hot stretch-heat set process, stillfind utility as the microporous films of this invention.

The term cold stretching as used herein is defined as stretching ordrawing a film to greater than its original length and at a stretchingtemperature, i.e., the temperature of the film being stretched, lessthan the temperature at which the melting of the film begins when thefilm is uniformly heated from a temperature of 25 C. at a rate of C. perminute. The terms hot stretching or hot stretching-heat setting as usedherein is defined as stretching above the temperature at which meltingbegins when the film is heated from a temperature of C. at a rate of 20C. per minute, but below the normal melting point of the polymer, i.e.,below the temperature at which fusion occurs. For example, usingpolypropylene elastic film, cold stretching is carried out preferablybelow about 120 C. while hot stretching or hot stretching-heat settingis carried out above this temperature.

When a separate heat setting step is employed it follows the coldstretching-heat stretching steps and is carried out at from about 125 C.up to less than the fusion temperature of the film in question. Forpolypropylene the range preferably is about 130 C. to 160 C.

The total amount of stretching or drawing which should occur when eithera single stretching or consecutive stretching steps occur is in therange of about 10 to about 300 percent of the original length of thefilm prior to stretching.

The resulting microporous film exhibits a final crystallinity ofpreferably at least percent, more preferably about to 100 percent asdetermined by the aforementioned X-ray method and as previously definedan elastic recovery from a 50% strain of at least 50% preferably toFurthermore, this film exhibits an average pore size of about to 12,000angstroms more usually 150 to 5,000 angstroms, the values beingdetermined by mercury porosimetry as described in an article by R. G.Quynn et al. on pages 21-34 of Textile Research Journal, January 1963.

As hereinabove indicated, the subject invention relates to the use of aunique open-celled microporous film as described above and in US. Ser.No. 876,511 filed Nov. 13, 1969, now abandoned and incorporated byreference, which application is assigned to the assignees of the instantinvention, as an improved, highly efiicient desiccant/fumigant packagingmaterial. The following examples are illustrative of the desirablecharacteristics possessed by this film and are not intended to limit thepresent invention in any manner.

EXAMPLE I Crystalline polypropylene having a melt index of 0.7 and adensity of 0.92 is melt extruded at 230 C. through an 8 inch slit die ofthe coat hanger type using a 1 inch extruder with a shallow meteringscrew. The length to diameter ratio of the extruded barrel is 24/ 1. Theextrudate is drawn down very rapidly to a melt drawdown i0 ratio of 150,and contacted with a rotating casting roll maintained at 50 C. and 0.75inch from the lip of the die. The film produced in this fashion is foundto have the following properties: thickness, 0.001 inch, recovery from50 percent elongation at 25 C., 50.3 percent, crystallinity, 59.6percent.

A sample of this film is oven annealed in air with a slight tension atC. for about 30 minutes, removed from the oven and allowed to cool. Itis then found to have the following properties: recovery from a 50percent elongation at 25 C., 90.5 percent; crystallinity 68.8 percent.

The anneaied elastic film is first cold stretched at 25 C. andthereafter hot stretched at C. Total stretch is 100 percent, based onthe original length of the film, and the extension ratio is 0.90:1. Thefilm is then heat set under tension, i.e. at constant length at 145 C.on 10 minutes in air.

Porosity, tear, and tensile data of the above prepared film is asfollows:

Tear, gins! Break, p.s.i.

N flux L B W 4 mils L W P 0 (3 grams) is placed between two 2" x 2"sheets of the above-prepared microporous polypropylene and the edgesheat sealed. The pouch (total weight 4 gms.) is then placed upon a trayin a desiccator containing 50 cc. of water and the desiccator sealed.For comparison, 2 grams of P 0 is placed in a small open beaker within adesiccator also containing 50 cc. of water and the desiccator sealed.

After four hours the P 0 in the open beaker realizes a weight gain of 3gms. H O/ gm. P 0 and the P 0 in the completely sealed pouch gains 1 gm.H O/ gm. P 0

EXAMPLE H A small open dish containing 7.3 gms. P 0 is placed in a roomwhich has a relative humidity of about 65 percent. Also placed in theroom is a 2" x 2" pouch, prepared as in Example I containing 4.8 gms. P0

After four hours the P 0 in the open beaker realizes a weight gain of 2gm. H O/gm. P 0 and the P 0 in the completely sealed pouch gains 0.225gm. moisture/ gm. P 0

EXAMPLE III Three pint bottles are /3 filled with water-saturatedbenzene (0.05%). Into each bottle is placed a 2" x 2" pouch containing 2gms. of P 0 prepared as in Example I.

At the end of the first hour, the pouch is removed from bottle #1. Atthe end of the third hour, the pouch is removed from #2 and at the sixthhour, from bottle #3. The water content in bottle #1 is reduced to0.03%; in bottle #2 to 0.02%; and in bottle #3 to 0.02%.

EXAMPLE IV The film forming polymer of this example, crystallinepolyethylene having a density of 0.96 and a melt index of 0.7, is meltextruded at C. through a 4 inch diameter annular die having an openingof 0.04 inch. The hot tube thus formed is expanded 1.5 times by internalair pressure and cooied by an air stream impinging on the film from anair ring located around and above the die. The extrusion is accomplishedwith an extruder of 24:1 length to diameter ratio and a shallow channelmetering screw. The extrudate is drawn down to a drawdown ratio of 10021and passed through a series of rollers which collapses the tube. Afterwind-up the film is oven annealed in a tensionless state at 115 C. for16 hours.

After removal from the oven, the film is allowed to cool, and stretchedat an extension ratio 015 0.80, by 50 percent of its original lengthwith cold stretching being conducted at 25 C. and hot stretching beingconducted at 12 EXAMPLE II The open-cell microporous film of the aboveExample I was mechanically cross-laminated to itself, i.e., the machinedirection axes were 90 apart. The film laminate 115 C., and heat set inthe oven at constant len h for minutes at 120 C., after which it isfound to hi e the 5 yielded the followmg date pagan-celled microporousstructure of the present inven- Teangms. Thick Three bottles as inExample III are filled with ben- Ngfilx L W mils L w Z6116 containing0.06% water and a heat sealed pouch Of 30-60 120g 1 203 2 0 23 000 3 000the above prepared microporous film which is mechani- 1 callycross-laminated to itself, i.e., the machine direction mmum Va axes are90 apart, containing 3 grns. P 0 is placed in EXAMPLE In each- W thepouches are removed from bottles 3 Microporous polypropylene film havinga total surface and 4 :dfter hours pf f y Karl P1861161 area per gram of40 square meters, a volume of space per l/ of the {emalnmg benzene W111Yleld the followlng volume of material of 0.5 cubic centimeters per gram2 concentfatlollsl and a N flux value of 113 was contacted with poly- T320 propylene film which had been pin punched to yield a 36 mmfiltration percent open area with 225 holes per square inch. The

Home number (hrs') (percent) microporous polypropylene film and the pinpunched g 2? polypropylene film were then put through an embossing 6 Mroll revolving at 8 revolutions per minute at 80 C. and

24 (1005 a pressure of 475 pounds per square inch. The physical A L Vproperties of the resultant product is compared with A four gram chargeof paradichlorobenzene is placed 25 the apovc descnbed nficrqporouspolylimpylene film with between two X sheets of the micropomus film nomicroporous backing 1n the followlng table: duced in Example IV and theedges completely heat sealed. Four Black Carpet Beetles and theabove-identified Film gfi fig z luv/13R; Tear s lgfflf pouch are placedin a metal test chamber of 25 liter catpacity. Within 100 hours there isa 100% mortality i ff gg gj j 2g ,3 ra e.

The following examples are illustrative of the desirable fggt ggiggggfor 10 pass 1 area at Pressure equal sterility-barrier characteristicspossessed by this film and gMeasuredingramsper24hwrspersquaremetersperASTM 15-96-60. are not intended to limit the present invention in any4%22253 lfififi fiiifig tt 83833;; manner.

EXAMPLE I Th f ll l Crystalline polypropylene having a melt index of 0.7e o owmg Pat ogemc organisms. were p acted and a density of 0.92 is meltextruded at 230 C. through tweeln layers of mlcroporous lammate film mExan 8 inch slit die of the coat hanger type using a 1 inch 40 amp eextruder with a shallow metering screw. The length to (1) Sta h [000cmre 0 g fg diameter ratio of the extruded barrel is 24/1. The ex- (2) E ss 0 5 6 1' trudate is drawn down very rapidly to a melt drawdown 3 "K"?ratio of 150, and contacted with a rotating casting roll Omen aserugmosa maintained at 50 C. and 0.75 inch from the lip of thePMGIOCOCCEIS die. The film produced in this fashion is found to have (5)rate, guns X15 the following properties: thickness, 0.001 inch, recoveryA totally enclosed pouch was formed by impulse sealing from percentelongation at 25 C., 50.3 percent, the edges of the layers. The packagewas placed within crystallinity, 59.6 percent. a Castle Ethylene-oxideSterilizer and a 23" vacuum ap- A sample of this film is oven annealedin air with a 50 plied. This was immediately followed by a 8-10 poundslight tension at 140 C. for about 30 minutes, removed pressurization at130 F. with a 12% ethylene oxide/ 88% from the oven and allowed to cool.It is then found to trichlorofiuoromethane gas mixture for 4 hours. Whenthe have the following properties: recovery from a 50 percent bag wasremoved from the sterilizer, small amounts of elongation at 25 C., 90.5percent; crystallinity 68.8 pergas residual evaporated quickly. Theorganisms inside cent. were completely destroyed. Sterile conditionsexisted in- The annealed elastic film is first cold stretched at 25 sideof this bag for a period exceeding 90 days (end of C. and thereafter hotstretched at 145 C. Total stretch test period) during which time thesterile pouch was is 100 percent, based on the original length of thefilm, repeatedly submerged in water. and the extension ratio is 0.90:1.Nitrogen flux (at X M C.) of the resulting open-celled microporous filmis 60 E PLE v 125.5 10- gram moles per cmF-min. A steile pouch wasformed as in Example IV an the Porosity, tear and tensile data of theabove prepared outside was coated with the pathogenic organisms of thatfilm are compared with data from commercially availexample in a.thioglycolate medium in an attempt to deable sterile packaging films inthe following table: termine if bacteria could grow through themicroporous Tear, grns. Break, p.s.i. Thick, Sterile packaging materialN2 flux 1 L 4 W 15 mils. L W

MPF -130 1-2 1.0 20,000 3,400 Polyethylene film (Bard-Parker) 0 121 963. 2 950 544 Polyester-polyethylene film (Pharmaseal) 0 5 5 1.6 8,82010,400 Nylon film (Tower Pks.) 0 153 10 1. 5 8, 45040, 000

l Microporous film of the present invention. 1 NXIO- g. moles Nz/cm.min. at 200 p.s.i.g. difierential pressure.

3 Notched Elmendorf Tear ASTM 13-1922-67 constant radius specimen:

laminate. The bag was maintained at 37 C. and 90100% humidity for 3days. When the pouch was removed from this controlled climate, theorganisms had not penetrated the pouch, i.e., sterile conditionscontinued to exis wihin the pouch.

EXAMPLE VI A totally enclosed pouch was formed about apolyethylene-rubber catheter by impulse sealing two layers of theopen-cell microporous polypropylene laminate film of Example III. Thispackage was placed within a Castle Ethylene-oxide Sterilizer and a 23"vacuum applied. This was immediately followed by an 8 10 poundpressurization at 130 F. with a 12% ethyleneoxide/ 88%trichlorofiuoromethane gas mixture for 4 hours. The package was thenaerated for 12 hours. At the end of this time, the residualethylene-oxide in the polyethylene-rubber catheter was below the levelwhich would prove to be an irritant to human body tissue.

For comparative purposes, a similar test was conducted using anon-porous polypropylene film package. To effect sterilization, asterilant exposure time of 12 hours was necessary. To then reduce theabsorbed ethylene-oxide concentration in the polyethylene-rubbercathether to a level below that which would be an irritant to human bodytissue required a combined aeration and storage time of 2 days.

EXAMPLE VII The open-cell microporous laminate of Example III was testedfor resistance to pressure up to 50 lbs. with dry and moist air. Nopenetration was noticed with a fog the size of about 1 micron.

EXAMPLE VIII A package such as that in Example IV containing identicalpathogenic organisms was placed in a steam autoclave at 250-270 F. for 4hours. The organisms were completely destroyed and a small amount ofresidual moisture inside of the bag after removal from the autoclaveevaporated within minutes. The package experienced a small dimensionalshrinkage of from about 5 to with a not inconsiderable reduction inporosity but the interior of the package remained sterile for a periodexceeding 90 days during which time the package was repeatedly submergedin water.

EXAMPLE IX Crystalline 4-methyl-1-pentene having a melt index of 1.5 wasmelt extruded at 250270 C. through an 8 inch slit die of the coat hangertype using a 1 inch extruder with a shallow metering screw. The lengthto diameter ratio of the extruded barrel is 24/1. The extrudate wasdrawn down very rapidly to a melt drawdown ratio of 150, and contactedwith a rotating casting roll maintained at 140 C. and 0.75 inch from thetip of the die. The film produced in this fashion is found to have thefollowing properties: thickness, 0.001 inch; recovery from 50 percentelongation at 25 C., 87 percent.

A sample of this film is oven annealed in air with a slight tension at160 C. for about minutes, removed from the oven and allowed to cool. Itis then found to have the following properties: recovery from a 50percent elongation at C., 92 percent.

The annealed elastic film is first cold stretched at 25 C. andthereafter stretched at 145 C. Total stretch is 100 percent, based onthe original length of the film, and the extension ratio is 0.9.Nitrogen flux (at 65 C.) of the resulting open-celled microporous filmis 25x10- gram moles per cm. min.

This film was then contacted with 4-methyl-l-pentene film which had beenpin punched to yield at 36 percent open area with 225 holes per squareinch. The microporous 4-methyl-1-pentene film and the pin punched 4-methyl1-pentene film were then put through an embossing roll revolvingat 8 revolutions per minute at 120 C. and a pressure of 250 pounds persquare inch. A pack age containing the pathogenic organisms listed inExample IV was formed as described in that example utilizing theabove-prepared laminated film. The package was placed in a steamautoclave at 250270 F. for 4 hours. The organisms were completelydestroyed; residual moisture within the package after removal from theautoclave evaporated within minutes; and there was less than 10%shrinkage of the package.

Illustrated above have been but a few of the many advantages inherent inthis unique sterile packaging material. As a result of the extremelyrapid gas transport through this film, it shows no tendency to ruptureeven under repeated sterilization to allow compacting and thereforeefficient utilization of storage space.

An additional advantage to the use of this open-cell microporous film insterile packaging lies in the fact that this anisotropic film with asmall tug (-1-2 pounds) perpendicular to the machine productiondirection will snap apart leaving a clean, lint-free split, i.e., noshredding occurs to contaminate the aseptic conditions within thepackage.

The principles, preferred embodiments and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however, is notto be construed as limited to the particular forms disclosed, sincethese are to be regarded as illustrative rather than restrictive.Variations and changes may be made by those skilled in the art withoutdeparting from the spirit of the present invention.

This four gram load corresponds to the dosage of 10 pounds per 1000cubic feet recommended by the US. Department of Agriculture.

What is claimed is:

l. In a package which allows the passage of chemical and sterilantvapors the improvement which comprises forming at least a portion of thepackage from a film consisting essentially of a polymer selected fromthe group consisting of polyolefins, polyacetals, polyamides,polyesters, polyalkylene sulfides and polyarylene oxides, wherein thefilm is further characterized by having a reduced bulk density ascompared to the bulk density of the corresponding polymer films havingno open celled structure, a crystallinity of above about 20 percent, apore size of about 100 to 12,000 angstroms, a nitrogen flux of about 5to 400, and an elastic recovery at 50 percent extension of greater than40 percent and wherein the film is further characterized by a pluralityof inter-connecting nonporous surface regions,

the non-porous surface regions being elongated and having their axes ofelongation substantially parallel; and a plurality of porous surfaceregions which include a plurality of fibrils, with the porous surfaceregions being defined by the non-porous surface regions,

the porous surface regions and the nonporous surface regionssubstantially alternating, and the fibrils being connected at each oftheir ends to the non-porous surface regions,

the fibrils being substantially parallel to each other and beingsubstantially perpendicular to said axes of elongation; and,

the fibrils defining pore spaces in the porous surface regions of thefilm.

2. The improved package according to claim 1 wherein the film has a bulkdensity of about 50 to percent of the bulk density of correspondingpolymer films having no open-celled structure.

3. The improved package according to claim 1 wherein the film has anitrogen flux of at least 40.

4. The improved package according to claim 1 wherein the film has asurface area of at least '30 sq. m./cc.

5. Th improved package according to claim 1 wherein the film has aperchloroethylene reaction value of zero or greater.

6. The improved package according to claim 1 wherein the film has anitrogen flux of 50 to 200.

7. The improved package according to claim 1 wherein the film has a bulkdensity of about 59 to 66 percent of the bulk density of a correspondingpolymer film having no open-celled structure.

8. The improved package according to claim 1 wherein the film has asurface area of 30 to 35 sq. m./cc.

9. The improved package according to claim 1 wherein the film has a bulkdensity of about 50 to 75 percent of the bulk density of correspondingpolymer films having no open-celled structure, a nitrogen flux of atleast 40, a surface area of at least 30 sq. m./cc., and aperchloroethylene reaction value of or greater.

10. The improved package according to claim 1 wherein the film has aperchloroethylene reaction value of 0 or greater, a nitrogen flux of 50to 200, a bulk density of 59 to 66 percent of the bulk density of thecorresponding polymer film having no open-celled structure, and asurface area of 30 to 35 sq. m./cc.

11. In a package which allows the passage of chemical vapors, theimprovement which comprises forming at least a portion of the packagefrom a film wherein said film consists essentially of a polymer selectedfrom the group consisting of polyolefins, polyacetals, polyamides,polyesters, polyalkylene sulfides and polyarylene oxides, wherein thefilm is further characterized by having a reduced bulk density ascompared to the bulk density of the corresponding polymer films havingno open-celled structure, a crystallinity of above about 30 percent, apore size of less than 5,000 angstroms, a nitrogen flux of greater than35.4, and a breaking elongation of 50 to 150 percent characterized by aplurality of interconnecting non-porous surface regions,

the non-porous surface regions being elongated and having their axes ofelongation substantially parallel; and a plurality of porous surfaceregions which include a plurality of fibrils, with the porous surfaceregions being defined by the non-porous surface regions,

the porous surface regions and the nonporous surface regionssubstantially alternating, and the fibrils being connected at each oftheir ends to the non-porous surface regions,

the fibrils being substantially parallel to each other and beingsubstantially perpendicular to said. axes of elongation; and,

the fibrils defining pore spaces in the porous surface regions of thefilm.

12. The improved package according to claim 11, wherein the film has abulk density of about 58 to 85 percent of the bulk density of thecorresponding polypropylene polymer films having no open-celledstructure.

13. The improved package according to claim 1,2 wherein the film has anitrogen flux of at least 40.

14. The improved package according to claim 12 wherein the film has asurface area of about 30 to 110 sq. m./cc.

15. The improved package according to claim 12 wherein the film has abulk density of about 62, a nitrogen fiux of about 100 and a surfacearea of about 60 sq. m./cc.

16. The improved package, according to claim 12 wherein the film has aperchloroethylene reaction value of 0 or greater.

17. The improved package according to claim 11 wherein the film has abulk density of about 58 to 85 percent of the bulk density ofcorresponding polypropylene films having no open-celled structure, anitrogen flux of at least 40, and a surface area of about 20 to 110 sq.m./cc.

18. In a sterilizable package the improvement which comprises forming atleast a portion of the package from a film consisting essentially of apolymer selected from the group consisting of polyolefins, polyacetals,polyamides, polyesters, polyalkylene sulfides and polyarylene oxides,wherein the film is further characterized by having a reduced bulkdensity as compared to the bulk density of the corresponding polymerfilms having no open celled structure, a crystallinity of above about 20percent, a pore size of about to 12,000 angstroms, a nitrogen flux ofabout 5 to 400, and an elastic recovery at 50 percent extension ofgreater than 40 percent and wherein the film is further characterized bya plurality of inter-connecting non-porous surface regions,

the non-porous surface regions being elongated and having their axes ofelongation substantially parallel; and a plurality of porous surfaceregions which include a plurality of fibrils, with the porous surfaceregions being definedby the non-porous surface regions,

the fibrils being substantially parallel to each other and beingsubstantially perpendicular to said axes of elongation; and,

the fibrils defining pore spaces in the porous surface regions of thefilm.

19. In a package for enclosing chemical compounds selected from thegroup consisting of fumigants and desiccants, the improvement whichcomprises forming at least a portion of the package from a filmconsisting essentially of a polymer selected from the group consistingof polyolefins, polyacetals, polyamides, polyesters, polyalkylenesulfides and polyarylene oxides, wherein the film is furthercharacterized by having a reduced bulk density as compared to the bulkdensity of the corresponding polymer films having no open celledstructure, a crystallinity of above about 20 percent, a pore size ofabout 100 to 12,000 angstroms, a nitrogen flux of about 5 to 400, and anelastic recovery at 50 percent extension of greater than 40 percent andwherein the film is further characterized by a plurality ofinter-connecting non-porous surface regions,

the non-porous surface regions being elongated and having their axes ofelongation substantially parallel; and a plurality of porous surfaceregions which include a plurality of fibrils, with the porous surfaceregions being defined by the non-porous surface regions,

the porous surface regions and the non-porous surface regionssubstantially alternating, and the fibrils being connected at each oftheir ends to the non-porous surface regions,

the fibrils being substantially parallel to each other and beingsubstantially perpendicular to said axes of elongation; and,

the fibrils defining pore spaces in the porous surface regions of thefilm.

References Cited UNITED STATES PATENTS 3,100,733 8/1963 Bundy 161-4593,122,141 2/1964 Crowe 161-459 3,378,507 4/1968- Sargent et al 161-1593,426,754 2/1969 Bierenbaum et a1. 161159 3,578,544 5/1971 Thorsrud161-159 3,578,545 5/1971 Carson et a1 161--159 MORRIS SUSSMAN, PrimaryExaminer US. Cl. X.R.

